Cisterna magna or Arachnoid cyst in Fetus Brain

My Wife is in 7th month of Pregnancy. She had anomaly Scan of fetus and found that there is Giant Cisterna Manga in brain and it is about 1.4cm in diameter. First Radiologist identified it as Arachnoid cyst but Second Radiologist (he seems to be right) identified it as Cisterna Manga. We are much worried about this finding. We like to asks following questions form worthy experts on this forum:-
1. What are the causes of these cysts?
2. Are these cysts harmful?
3. What are their side effects after birth?
4. Do we have some treatment before birth or after birth?
5. Can a person live a complete or normal life with these cysts?
thanks

[Arti Sharma, M.D.]

Hi there,
Arachnoid cysts are cerebrospinal fluid-filled sacs located between the brain or spinal cord and the arachnoid membrane .Mega-cisterna magna is an enlarged cisterna magna with an intact Cerebellar vermis and fourth ventricle. In this condition the fourth ventricle, vermis, and Cerebellar hemispheres are normally formed. Mega cisterna magna is considered to be a normal variant which may be  corresponding to an expansion of the inferior cerebellar lack or big cisterna, with free communication with peri-medullar arachnoidal space. In few cases an enlarged cisterna magna in the third trimester can be  a clue to fetal trisomy 18.It is best to discuss the test results with a neonatologist. Do keep me posted.
Best luck and regards!